Dielectric Stack Filters for Ex Situ and In Situ UV Optical-Fiber Probe Raman Spectroscopic Measurements

Dielectric stack interference ® lters can be used in conjunction with a high-throughput single-stage spectrograph to facilitate the measurement of high signal-to-noise (S/N) ultraviolet (UV) Raman spectra with 228.9-nm and 244-nm excitation wavelengths. Placed between the sample and the spectrograph, these ® lters re ̄ ect Rayleigh scattering while transmitting Stokes-shifted Raman scattering. We have measured UV Raman bands from solid, highly scattering samples down to a 290-cm 2 1 shift from the Rayleigh line. The high throughput of the ® ltered single-stage spectrograph enables the measurement of UV Raman spectra from photo-labile samples, including DNA and the energetic materials pentaerythritol tetranitrate (PETN) and trinitrotoluene (TNT), with suf® ciently low excitation powers and short accumulation times to minimize photo-alteration. High S/N UV preresonance and resonance Raman are obtained for PETN and TNT within 1 s, indicating the possible application of UV Raman spectroscopy as a rapid, highly selective screening methodology for the detection of trace levels of contraband explosives. Furthermore, the incorporation of these dielectric ® lters within a UV optical-® ber Raman probe head provides simultaneous Rayleigh rejection and removal of background silica Raman scattering. With the use of a 244-nm UV optical-® ber probe, we measured Raman spectra from 100 nM to 10 m Mconcentrations of polycyclic aromatic hydrocarbon (PAH) in water, even in the presence of an equimolar concentration of the visible ̄ uorophore rhodamine 6G (R6G). Thus, we demonstrate the potential of UV Raman optical-® ber probes for minimally invasive in situ real-time monitoring at low analyte concentrations and within environments in which ̄ uorescence backgrounds would prevent measurements with visible Raman optical-® ber probes.